Mass transfer, whether it be dialysis, oxygenation, desalination, or heat exchange, is basically a surface phenomenon between two fluids and the transfer barrier there between. Inefficiency occurs in mass transfer devices due to poor transfer media, non-turbulent fluid flow, poor mixing, poor surface contact between fluids and the transfer barrier and non-uniform distribution of fluid on one or both sides of the transfer barrier. In multiple layer devices, non-uniform distribution among layers impairs efficiency. In general, most mass transfer devices are designed to minimize the enumerated factors in order to maximize mass transfer. Depending upon the mass transfer involved, trade-offs are required to achieve the desired effect.
Some uses for mass transfer devices are as dialyzers to effect solute and solvent transfers between dialysand and dialysate; as hemodialyzers to effect solute and water transfer between blood and dialysate; as oxygenators to effect carbon dioxide, oxygen, and nitrogen transfers between blood and oxygen containing gases; as heat exchangers to effect heat transfer between fluids such as in automobile radiators, in refrigeration units, in room heating units, and in solar heating devices; as reverse osmosis devices in which pressure gradients serve as the driving force to separate solute and solvent, as in desalination of water; and as filtration devices in which pressure gradients are used to separate solids and liquids. Each of these mass transfer devices has its peculiar needs; however, certain characteristics are desirable in all devices. Desirable characteristics include: (1) High mass transfer coefficients, which inturn require thin fluid films, uniform fluid distribution, good mixing of fluids, and high flux transfer membranes; (2) Adjustable flow pressure gradients; (3) Adjustable transmembrane pressure gradients, in the case of dialyzers and filtration devices; and (4) Compactness of design. In certain uses, such as hemodialyzers and blood oxygenators, perfect fluid channel seals are essential, since leaks could be fatal.
In both of my prior art U.S. Pat. Nos. 3,522,885 and 3,565,258, respectively, issued on Aug. 4, 1970 and Feb. 23, 1971, I utilized parallel flow, mass transfer devices for hemodialysis. In both of those devices, solute transfer rates necessary to perform adequate hemodialysis within a reasonable period of time could not be obtained. The latter device also suffered a progressive decrease in solute transfer rate which was attributable to build-up of proteinaceous material on the blood side of the transfer membrane.
Inefficiency of these device was mistakenly attributed to design of the membrane support structure. However, several variations of membrane support design, substituting for the original netting, did not alter dialyzer efficiency. The reason for poor performance was identified as non-uniform flow distribution both between layers and within individual layers. Poor distribution was responsible for low mass transfer and proteinaceous build-up on the membranes. Maldistribution in turn was secondary to inadequate entry and exit manifolding on individual plates, to inadequate manifolding of plates in the stack, and to distortion of fluid channel dimensions by pressure differentials in the cross-flow design configuration.
The Alwell et al. U.S. Pat. No. 3,511,381, issued May 12, 1970, for DIALYSIS BLOOD DISTRIBUTION GROOVES, is representative of a type of dialyzer which uses two membranes between adjacent supports to provide a flow path for blood while dialysate flows between the membrane and the adjacent support. In this type of construction, both the dialysate and the blood are introduced in a direction normal to the fluid flow during mass transfer, whereby both the blood and the dialysate pass through apertures in the supports and membranes. This construction results in difficult sealing problems as well as making the device expensive to produce.
U.S. Pat. No. 3,547,271, issued to Edwards, Dec. 15, 1970, for MEMBRANE FLUID DIFFUSION EXCHANGE DEVICE, is representative of another type of mass transfer device, that is an oxygenator in which adjacent membranes provide one fluid channel and the other fluid channel is provided by a support and a membrane. The Edwards construction runs afoul of the same problem as the Alwell et al. construction with difficult sealing problems.
The Alwell et al. U.S. Pat. No. 3,516,548, issued June 23, 1970, for DIALYSIS MEANS HAVING SPACING DISKS WITH GRATINGS DISPLACED OR TWISTED IN RELATION TO EACH OTHER, recognizes that the fluid distribution devices of the previously discussed Alwell et al. patent are expensive and difficult to use, the U.S. Pat. No. 3,516,548 patent being directed toward a less expensive fluid distribution device. Nevertheless, the use of fluid distribution system wherein fluid passes through apertures in membranes and supports is frought with possibilities of leakage and is expensive to manufacture.
The Esmond U.S. Pat. No. 3,738,495, issued June 12, 1973, for EXCHANGE DEVICE, illustrates another critical defect in the prior art. In the Esmond device, each cover plate has a manifold for distributing fluids and each flow plate has a manifold for distributing fluids, but the manifold designs make even flow distribution difficult to achieve.
Many of the prior art devices direct fluid entry and exit through holes pierced in the membranes and supporting structure. Membrane piercing increases handling and construction costs and increases the possibility of leakage during operation. Still others of these devices require that the two fluids be distributed along non-identical paths, whereby most of the thin film contact area is lost. Still other of these devices do not provides adequate fluid manifolding, whereby fluid distribution is insufficient to achieve proper mass transfer rates. Some of these devices require large fluid priming volume which is undesirable in certain mass transfer operations such as dialysis. Others of these devices are not readily adaptable to provide either small or large pressure drops across the device or variable pressure drops, and no prior art device known can easily accommodate all of these requirements. Finally, some prior art devices simply do not provide the requisite surface area for the fluids to contact in order to accomplish the desired mass transfer rate.
In short flow path mass transfer devices, entrance and exit effects predominate and these effects must be precisely controlled to provide uniform fluid flow to each plate and along each plate. There are two distinct problems, one being to provide uniform fluid distribution to each plate in a stack of plates and the other being to provide uniform fluid distribution across and along each plate. Heretofore, it has been assumed by investigators in short flow path mass transfer devices that membrane support design was critical, but, I have found that entrance and exit effects predominate and are much more critical. It is the flow distribution and thin boundary layers which primarily determine the extent of mass transfer, and in short parallel path devices the entrance and exit effects control the flow distribution.
By providing unique headers, fluid distribution to and from each plate is uniform and by providing unique plate manifolds, uniform fluid distribution along each plate is attained. The particular design provides maximum membrane support resulting in high burst strength for the membrane, an important feature in dialyzer or oxygenator designs. Also, internal resistance to fluid flow is a design parameter which can be altered to provide a wide range of pressure drops across the device, large pressure drops for reverse osmosis and low pressure drops for dialyzers.